Pond turnover is a natural process in ponds. It involves the mixing of different layers of water that have separated due to temperature differences. This phenomenon is a regular part of a pond’s annual cycle.
Why and How Ponds Turn Over
Pond turnover is driven by changes in water temperature and density across seasons. During warmer months, a pond often develops distinct layers, known as thermal stratification. The sun heats the surface water, creating a warmer, less dense upper layer, while cooler, denser water settles at the bottom. This stratification also leads to different oxygen levels, with the surface layer rich in oxygen and the bottom layer becoming oxygen-depleted due to decomposition.
As autumn arrives, the surface water begins to cool, increasing its density. Water reaches its maximum density at approximately 4 degrees Celsius (39.2 degrees Fahrenheit). When the surface water cools to a temperature similar to the bottom layer, the density difference diminishes. This allows the entire water column to mix, often aided by wind, causing the pond to “turn over.” A similar process can occur in spring as ice melts and surface water warms to 4 degrees Celsius, then becomes less dense, leading to mixing.
What You See and Smell During Turnover
Changes in water appearance are common during a pond turnover. The mixing process stirs up sediments and organic matter that have settled at the bottom, making the water murky or cloudy. Water color might shift to gray or brown as these materials redistribute throughout the pond.
An unpleasant odor is another common sign of pond turnover. This smell is often described as a sulfurous or “rotten egg” scent. This odor results from hydrogen sulfide gas, which accumulates in oxygen-poor bottom layers from organic decomposition. When layers mix, these trapped gases release into the atmosphere.
Effects on Aquatic Life and Pond Health
Pond turnover affects the aquatic ecosystem by redistributing oxygen and nutrients. During stratification, the bottom layer of a pond can become depleted of dissolved oxygen due to organic matter decomposition. When turnover occurs, this oxygen-poor water from the bottom mixes with the more oxygenated surface water.
This decrease can stress fish and other aquatic organisms, potentially causing mortality if oxygen levels become too low. Fish may be seen at the surface, gasping for air, indicating insufficient oxygen in the water column.
Beyond oxygen, turnover also redistributes nutrients like phosphorus and nitrogen from the pond bottom to the surface. While natural, this influx into upper, sunlit layers can promote rapid algal growth. This can result in an algal bloom, further impacting water quality by consuming oxygen as algae die and decompose.